Stainless steel sheathed thermocouples are commonly used to measure the temperature of a material in environment of high thermal radiation. It is known that measurement errors inherent in such thermocouple devices can be reduced by coating the sheath of the thermocouple with gold ink which has a very low thermal emissivity. To this end, various methods have been developed for coating a thermocouple sheath with gold ink.
Briefly summarizing the steps of an exemplary prior art coating method, the thermocouple sheath is hand-painted with gold ink, then heated and cured at over a 1000.degree. F. for one hour, and finally hand polished. The steps of hand-painting, curing and hand-polishing are typically repeated at least once.
This prior art method suffers a number of disadvantages and has been found to be generally unsatisfactory. For example, hand-painting and hand-polishing of the thermocouple sheath is time-consuming and costly. More importantly however, hand-polishing is inadequate to effectively prepare the surface of the thermocouple sheath for subsequent application of gold ink. As a result, a gold ink coating thus applied is not sufficiently smooth and shiny, and, since reflectivity is important for minimizing thermocouple measurement errors, the performance of the associated thermocouple is thereby limited. Further, as a result of ineffective hand-polishing, the gold ink coating deteriorates quickly when exposed to temperatures above 1000.degree. F. and becomes blackened and ineffective after only a few hours of use.